ldbl-128ibm-compat: Provide a scalb implementation

Reuse the template in order to provide the redirect for
scalbl to __scalbieee128, but avoid any extra aliasing
as this is intended to support long double redirects only.

sysdeps/ieee754/ldbl-128ibm-compat/Versions

@@ -90,6 +90,7 @@ libm {
     __rintieee128;
     __roundevenieee128;
     __roundieee128;
+    __scalbieee128;
     __scalblnieee128;
     __scalbnieee128;
     __setpayloadieee128;

sysdeps/ieee754/ldbl-128ibm-compat/e_scalbf128.c

@@ -0,0 +1,26 @@
+/* Multiply _Float128 by integral power of 2
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <math-type-macros-float128.h>
+
+/* *_finite symbols are deprecated since this layer was introduced.
+   No symbols are needed. */
+#undef declare_mgen_finite_alias
+#define declare_mgen_finite_alias(from, to)
+
+#include <math/e_scalb_template.c>

sysdeps/ieee754/ldbl-128ibm-compat/w_scalbf128.c

@@ -0,0 +1,27 @@
+/* Multiply _Float128 by integral power of 2
+   Copyright (C) 2020 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <float128_private.h>
+#include <math-type-macros-float128.h>
+
+#undef declare_mgen_alias
+#define declare_mgen_alias(a,b)
+#define __ieee754_scalbl __ieee754_scalbf128
+#include <w_scalb_template.c>
+
+libm_alias_float128_other_r_ldbl (__scalb, scalb,)